Active input voltage sensing for low voltage analog signal detection

Abstract

An input circuit for detecting low voltage analog signals in an electrically noisy environment receives the analog input signal at an input terminal. The analog input signal is compared to a variable reference signal at a comparator circuit. An active hysteresis circuit provides feedback to the comparator. In a first operating mode, the active hysteresis circuit may be disabled or be configured to output a constant voltage. In a second operating mode, the active hysteresis circuit may be enabled or configured to output a varying level of voltage. The output of the feedback circuit is summed with the variable reference signal and supplied as the input signal to the comparator, such that signal against which the analog input signal is compared is a general constant value in the first operating mode and varies with respect to time in the second operating mode.

We claim: 
     
       1. An input circuit for detecting an analog signal, the input circuit comprising:
 a terminal configured to receive the analog signal at the input circuit; 
 a variable reference signal operative to equal a first reference signal in a first operating mode and a second reference signal in a second operating mode, wherein the first reference signal is a different magnitude than the second reference signal; 
 a comparator circuit including a first input, a second input, and an output, wherein the second input is operative to receive the analog signal; and 
 an active hysteresis circuit including an input and an output, wherein:
 the input of the active hysteresis circuit is configured to receive the output from the comparator circuit, 
 the output of the active hysteresis circuit is added to the variable reference signal, 
 the first input of the comparator circuit is operative to receive the variable reference signal added to the output of the active hysteresis circuit, 
 the output of the active hysteresis circuit is a constant value in the first operating mode, and 
 the output of the active hysteresis circuit is a varying value during the second operating mode. 
 
 
     
     
       2. The input circuit of  claim 1  further comprising:
 a processor operative to output a digital signal corresponding to a desired reference signal; and 
 a digital to analog converter having an input and an output, wherein the input is configured to receive the digital signal from the processor and the output is the variable reference signal. 
 
     
     
       3. The input circuit of  claim 2  wherein the desired reference signal is a first constant value corresponding to the first reference signal in the first operating mode. 
     
     
       4. The input circuit of  claim 2  wherein the desired reference signal is a second constant value corresponding to the second reference signal in the second operating mode. 
     
     
       5. The input circuit of  claim 1  wherein the output of the active hysteresis circuit is a function of a resistive-capacitive circuit to generate the varying value during the second operating mode. 
     
     
       6. The input circuit of  claim 1  wherein the output of the comparator circuit is a first value when the first input signal is greater than the second input signal and the output of the comparator circuit is a second value when the first input signal is less than the second input signal. 
     
     
       7. The input circuit of  claim 6  wherein the first operating mode is selected when the output of the comparator is the first value and the second operating mode is selected when the output of the comparator is the second value. 
     
     
       8. The input circuit of  claim 1  wherein the input circuit is configured to receive at the terminal of the input circuit a time variant analog signal with an amplitude that varies periodically between a maximum value and a minimum value with respect to time and wherein the variable reference signal is the first reference signal during a first half cycle of the time variant analog signal and the variable reference signal is the second reference signal during a second half cycle of the time variant analog signal. 
     
     
       9. A method of detecting an analog signal with an input circuit, the method comprising the steps of:
 receiving the analog signal at a terminal of the input circuit; 
 generating a variable reference signal with the input circuit, wherein the variable reference signal is a first reference signal in a first operating mode and a second reference signal in a second operating mode, wherein the first reference signal is a different magnitude than the second reference signal; 
 disabling an active hysteresis circuit in the input circuit during the first operating mode; 
 enabling the active hysteresis circuit in the input circuit during the second operating mode, wherein:
 the active hysteresis circuit includes an input and an output, 
 the input of the active hysteresis circuit is configured to receive an output of a comparator circuit, and 
 the output of the active hysteresis circuit is added to the variable reference signal during the second operating mode; 
 
 comparing the analog signal to the variable reference signal at the comparator circuit in the first operating mode, wherein the variable reference signal is a first input signal and the analog signal is a second input signal to the comparator circuit in the first operating mode; and 
 comparing the analog signal to a sum of the output of the active hysteresis circuit and the variable reference signal at the comparator circuit in the second operating mode, wherein the sum of the output of the active hysteresis circuit and the variable reference signal is the first input signal and the analog signal is the second input signal to the comparator circuit in the second operating mode. 
 
     
     
       10. The method of  claim 9  wherein the step of disabling the active hysteresis circuit includes generating a constant output value from the active hysteresis circuit in the first operating mode, the method further comprising the step of adding the output of the active hysteresis circuit to the variable reference signal during the first operating mode and, wherein the sum of the output of the active hysteresis circuit and the variable reference signal is compared to the analog signal in the first operating mode. 
     
     
       11. The method of  claim 9  wherein the output of the active hysteresis circuit is a varying value during the second operating mode. 
     
     
       12. The method of  claim 11  wherein the varying value is a function of a resistive-capacitive circuit. 
     
     
       13. The method of  claim 9  wherein the output of the comparator circuit is a first value when the first input signal is greater than the second input signal and the output of the comparator circuit is a second value when the first input signal is less than the second input signal. 
     
     
       14. The method of  claim 13  further comprising the steps of:
 selecting the first operating mode when the output of the comparator is the first value, and 
 selecting the second operating mode when the output of the comparator is the second value. 
 
     
     
       15. An input module for an industrial controller, the input module comprising:
 a terminal configured to receive an analog signal; 
 a variable reference signal set to a first reference signal in a first operating mode and a second reference signal in a second operating mode, wherein the first reference signal is a different magnitude than the second reference signal; 
 a comparator circuit including a first input, a second input, and an output, wherein the second input is operative to receive the analog signal; and 
 an active hysteresis circuit including an input and an output, wherein:
 the input of the active hysteresis circuit is configured to receive the output from the comparator circuit, 
 the active hysteresis circuit is configured to generate an output signal having a constant value in the first operating mode, and 
 the active hysteresis circuit is configured to generate an output signal having a varying value in the second operating mode, and 
 the first input of the comparator circuit is configured to receive a sum of the variable reference signal and the output signal from the hysteresis circuit. 
 
 
     
     
       16. The input module of  claim 15  wherein the output signal of the active hysteresis circuit is a function of a resistive-capacitive circuit to generate the varying value during the second operating mode. 
     
     
       17. The input module of  claim 15  further comprising:
 a processor operative to output a digital signal corresponding to a desired reference signal; and 
 a digital to analog converter having an input and an output, wherein the input is configured to receive the digital signal from the processor and the output is the variable reference signal. 
 
     
     
       18. The input module of  claim 15  wherein:
 the industrial controller includes a processor module in communication with the input module, 
 the processor module has a processor operative to output a digital signal corresponding to a desired reference signal, and 
 the digital signal is transmitted from the processor module to the input module, the input module further comprising: 
 a digital to analog converter having an input and an output, wherein the input is configured to receive the digital signal from the processor module and the output of the digital to analog converter is the variable reference signal. 
 
     
     
       19. The input module of  claim 15  wherein the output of the comparator circuit is a first value when the first input signal is greater than the second input signal and the output of the comparator circuit is a second value when the first input signal is less than the second input signal. 
     
     
       20. The input module of  claim 19  wherein the first operating mode is selected when the output of the comparator is the first value and the second operating mode is selected when the output of the comparator is the second value.